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Stock and Dies.                            193

Fig. 205 ; and a left-handed turn will release the tool. When the
taper tap has done its work the middle tap is introduced in like
manner, carrying the operation a little further, and finally the plug
tap is passed through to give the finishing cut. After every
stroke forward, the workman releases the tool slightly, so as to
avoid undue pressure and perhaps breakage. (Seep. 1025.)

A. stock and dies is shewn at Fig. 206. A is the stock, pro-
vided with handles for turning, and B is an enlarged view of one
of the dies, having a thread upon it in reverse, and four cutting
surfaces at 90, two to each direction of rotation: so that the
thread may be cut both on advance and return. The dies are
shewn in position in the stock A, being dropped in at e and slid
along : then tightened by a tommy applied to the screw d. The
bolt to be screwed is first turned to the outside diameter of
the tap, and then fixed in the vice. The dies are separated
slightly, the stock brought over the bolt as at c, and the screw
advanced. The stock is now rotated until the length of the bolt
is traversed; then, on reversing the motion, a slightly increased
pressure given to the dies; and so the bolt is re-traversed again
and again, until so cut into by the dies as to show a perfect
thread, and gauge to proper diameter, which may be proved by
trying upon it the already tapped nut, and any degree of tightness
obtained after such trial. At each stroke a slight backward
release is given as before, and oil may he used as a lubricant
Various sized dies may also be applied to the same stock.

For screws under a \ in. diameter the Screw Plate in Fig.
207 replaces the stock and dies, and only one tap is required in-
stead of three.

The pitch of a screw being measured lefigthwise from centre to
centre of the threads, let us unwind the latter, both at the top and
bottom of the V groove. The diagram in Fig. 20$ will shew the
result obtained in each case, and it will be clearly seen that the
angle at the bottom of the thread is larger than that at its top.
But the action of the dies, in cutting, is to first mark out the top
of the thread with that part of the die formed to finish the angle
at the bottom, and it follows that by the time the thread is
finished, there will be an unnecessary endlong play of the bolt in
the nut. These faults are somewhat avoided by the use of the